Dealumination of zeolites: Part 4. Dealumination of offretite through hydrothermal treatment

Abstract

Nine samples of dealuminated offretite have been prepared by simple or double treatment, without addition of water, at high temperature (873–1123 K) of a KHOffretite sample (K 0.95H 3.45Al 4.4Si 13.6O 36). The samples were characterized by X-ray diffraction (unit cell parameters a and c, and “crystallinity” C RX), by 29Si n.m.r. (framework Si Al ratio and distribution of Al in T 1 and T 2 sites), by 27Al n.m.r. (framework and extraframework Al species), by i.r. spectroscopy (frequencies υ 1 and υ 2 of the asymmetric and symmetric TO bands), and by X.p.s. ( Si Al ratio of the outer surface). As with Y zeolites, highly dealuminated samples with a high C RX value can be obtained only through successive treatments with, intermediately, exchange of the K cations. The dealumination causes an increase in the Al T1 Al T2 ratio and of υ 1 and υ 2 but has practically no effect on the c parameter. Parameter a first decreases with the number of framework Al per unit cell (N Al), then remains quasi-constant for N Al ≤ 2.7. Nearly linear relationships are obtained between υ 1 and υ 2 and N Al, which allows an estimation of N Al to be made by i.r. spectroscopy.